I couldn't find any references to this study so apologize if this is a repeat. There was just a study published on 85,000 women in Norway that "found that those who started taking folic acid before getting pregnant were about 40 percent less likely to have a child who developed the disorder." Note that this was with the synthetic form of folic acid. Ian Lipkin was one of the authors. Correlation doesn't equal causation of course, but it does appear that there was a very significant effect. One article (and the study itself) point out that people with similar backgrounds often took fish oil, but that had no obvious effect on rates of autism.